(1) Field of the Invention
The present invention relates to a method and device for estimating an open circuit voltage of a battery, which supplies electric power to a load.
(2) Description of the Related Art
An open circuit voltage of a battery is determined by measuring a terminal voltage in its open circuit state of a battery in its equilibrium state and is a quantity reflecting a charging state of the battery. Therefore, an open circuit voltage of a battery is important in order to know how much electrical quantity remains in the battery (i.e. charging state) as to a battery in operation, which is loaded on a vehicle, for example.
In general, when a discharge current flows from a battery, a drop corresponding to the magnitude of the current takes place in a terminal voltage of the battery, on the other hand, when a charge current flows into a battery, the terminal voltage rises. For example, the voltage drop during discharge is caused by an internal impedance (combined resistance) of a battery and can be divided into a voltage drop (IR loss) due to a pure resistance (ohmic resistance) caused by structure of the battery, another voltage drop due to activation polarization of polarization resistance component caused by chemical reaction, and further voltage drop due to concentration polarization of the polarization resistance component caused by the chemical reaction.
When the discharge current becomes zero, the IR loss simultaneously becomes zero and the voltage drop due to the activation polarization also becomes zero in a relatively short period of time. The voltage drop due to the concentration polarization decreases as the discharge current decreases, however the voltage drop due to the concentration polarization remains for a relatively long period of time even when the discharge current becomes zero, because it takes a relatively long period of time for the concentration polarization to disappear by diffusion of electrolyte. Such a state in which the voltage drop due to discharge remains after completion of the discharge is called a non-equilibrium state. A terminal voltage of a battery measured in an open circuit state when the battery is in a non-equilibrium state is different from a terminal voltage measured when the battery is in an equilibrium state, that is open circuit voltage, in which the voltage drop due to the discharge completely disappears, and changes approaching to the open circuit voltage with time passing after the halt of the discharge.
In this connection, a terminal voltage of a battery, which rises as the charge current flows, is also different from an open circuit voltage, because the voltage rise due to the concentration polarization remains for a relatively long period of time similarly to the case of the discharge.
In general, a terminal voltage of a battery changes approaching to an open circuit voltage after completion of a charge as shown in FIG. 3 and changes approaching to an open circuit voltage after completion of a discharge as shown in FIG. 10. As shown in FIG. 11, after completion of a charge, for example, a time required to reach an equilibrium state is short when the temperature is high, while the time is long when the temperature is low.
Therefore, in general, a terminal voltage of a battery in an open circuit state has been measured when a certain time passes after completion of a charge or discharge, said certain time being considered to be a time required to reach an equilibrium state, which is 24 hours for example, and then thus measured terminal voltage has been regarded as an open circuit voltage.
In the method as described above, in order to obtain an open circuit voltage we have to wait for a certain period of time required for the system to reach a supposed equilibrium state after completion of a charge or discharge. Therefore, when a charge or discharge is started again before such a certain period of time required passes, there is no chance to obtain an open circuit voltage until a certain period of time passes again after the completion of such a next charge or discharge, causing a problem that there is very few chance for obtaining an open circuit voltage.
In addition, since the certain period of time required as described above varies depending upon temperature, if the terminal voltage of a battery in an open circuit state after a constant period of time passes after the completion of a charge or discharge is measured and regarded as an open circuit voltage regardless of ambient temperature, such a regarded open circuit voltage includes an error because the terminal voltage varies depending upon temperature, causing a problem that a compensation with respect to the temperature is required for canceling such an error.
It is therefore an objective of the present invention to solve the above problems and to provide a method and device for estimating an open circuit voltage of a battery, by which an open circuit voltage of a battery in operation can be relatively accurately estimated in a relatively short period of time after completion of a charge or discharge.
The present invention has been attained by finding out that a terminal voltage of a battery in an open circuit state after completion of a charge or discharge asymptotically approaches to a specific voltage, which can be estimated as an open circuit voltage according to a predetermined power approximation expression.
In order to attain the above objective, the present invention is to provide a method of estimating an open circuit voltage of a battery for supplying electric power to a load, comprising the steps of:
measuring and collecting a terminal voltage of the battery in an open circuit state in a specific cycle after completion of a charge or discharge of the battery;
computing a voltage value, to which a power approximation expression asymptotically approaches, as an assumed open circuit voltage of each time period from the collected terminal voltage, wherein the power approximation expression has an exponent of xe2x88x920.5 or about xe2x88x920.5 and is determined on the basis of the terminal voltage of each time period of a plurality of predetermined time periods; and
estimating the assumed open circuit voltage of a time period, in which its difference from the assumed open circuit voltage of a neighboring time period becomes minimum, as an open circuit voltage.
With the construction described above, after completion of a charge or discharge of the battery, an asymptote of a power approximation expression obtained by using the terminal voltage for a time period, in which the terminal voltages in accordance with the power approximation expression are relatively gather, from the terminal voltages measured in a relatively short period of time, can be estimated as an open circuit voltage of the battery. Therefore, a method of estimating an open circuit voltage of a battery, by which an open circuit voltage of a battery can be relatively accurately estimated in a relatively short period of time after completion of a charge or discharge, can be provided.
Preferably, each time period of a plurality of predetermined time periods is defined by a combination between one of a plurality of start time points and one of a plurality of finish time points, which are predetermined with an elapsed time after completion of a charge or discharge of the battery.
With the construction described above, an accurate open circuit voltage can be obtained.
Preferably, the shortest one of the start time points and the longest one of the finish time points correspond to a start and a finish of the time period, respectively, during which the terminal voltage of the battery in an open circuit state is measured and collected.
With the construction described above, an efficient measurement for obtaining an open circuit voltage can be carried out.
Preferably, an interval between a plurality of the start time points is smaller than an interval between a plurality of the finish time points.
With the construction described above, as for the components, which have no relation with the diffusion of the electrolyte, such as an internal resistivity, activation polarization, and overvoltage accompanied by gas generation, easily become the error factor, and are included in the voltage change right after a charge or discharge, it is possible to set the time periods finely.
Preferably, a time period, in which a value obtained by dividing a summation of absolute values of a difference from the assumed open circuit voltage of a neighboring time period by the number of the neighboring time periods becomes minimum, is set to be a time period, in which a difference from the assumed open circuit voltage of a neighboring time period becomes minimum.
With the construction described above, an open circuit voltage of a battery can be estimated by the relative comparison regardless of the number of the neighboring time periods.
Preferably, when the terminal voltage in an open circuit state is collected after completion of a charge, on the basis of a difference value between the terminal voltage in an open circuit state for each time period and an assumed open circuit voltage, a predetermined power approximation expression having a negative exponent is determined, and the determination of the power approximation expression is carried out repeatedly by updating the assumed open circuit voltage until the exponent of the determined power approximation expression becomes xe2x88x920.5 or about xe2x88x920.5, thereby computing a voltage value to which the power approximation expression asymptotically approaches.
With the construction described above, an asymptote of a power approximation expression for each time period is obtained as the assumed open circuit voltage for each time period in a relatively short period of time after completion of the charge of the battery.
Preferably, when the terminal voltage in an open circuit state is collected after completion of a discharge, on the basis of an absolute value of a value obtained by subtracting the assumed open circuit voltage from the terminal voltage in an open circuit state for each time period and on the basis of a difference value between the terminal voltage in an open circuit state for each time period and an assumed open circuit voltage, a predetermined power approximation expression having a negative exponent is determined, and the determination of the power approximation expression is carried out repeatedly by updating the assumed open circuit voltage until the exponent of the determined power approximation expression becomes xe2x88x920.5 or about xe2x88x920.5, thereby computing a voltage value to which the power approximation expression asymptotically approaches.
With the construction described above, an asymptote of a power approximation expression for each time period is obtained as the assumed open circuit voltage for each time period in a relatively short period of time after completion of the discharge of the battery.
Preferably, the power approximation expression is expressed by xcex1xc2x7tD, where t is time, xcex1 is an unknown coefficient, and D is an unknown negative exponent.
With the construction described above, an assumed open circuit voltage when the exponent D of the power approximation expression xcex1xc2x7tD becomes xe2x88x920.5 or about xe2x88x920.5 can be set as the assumed open circuit voltage for the time period.
Preferably, the number of values of the measured terminal voltage in an open circuit state for each time period is set to be an optional number equal to or more than 2 and the terminal voltage of the optional number is subjected to a regression calculation, thereby determining the exponent D of the power approximation expression.
With the construction described above, even if the exponent of the power approximation expression xcex1xc2x7tD does not become xe2x88x920.5, the assumed open circuit voltage of each time period can be obtained when the predetermined number of the determination of the power approximation expression is carried out.
In order to attain the above objective, the present invention is to provide a device for estimating an open circuit voltage of a battery for supplying electric power to a load, comprising:
measuring means for measuring a terminal voltage of the battery in an open circuit state in a specific cycle after completion of a charge or discharge of the battery;
collecting means for collecting the terminal voltage of the battery in an open circuit state, which is measured by the measuring means; and
computing means for computing a voltage value, to which a power approximation expression asymptotically approaches, as an assumed open circuit voltage of each time period from the terminal voltage collected by the collecting means, wherein the power approximation expression has an exponent of xe2x88x920.5 or about xe2x88x920.5 and is determined on the basis of the terminal voltage of each time period of a plurality of predetermined time periods,
wherein the assumed open circuit voltage of a time period, in which its difference from the assumed open circuit voltage of a neighboring time period becomes minimum, is estimated as an open circuit voltage.
With the construction described above, after completion of a charge or discharge of the battery, an asymptote of a power approximation expression obtained by using the terminal voltage for a time period, in which the terminal voltages in accordance with the power approximation expression are relatively gather, from the terminal voltages measured in a relatively short period of time, can be estimated as an open circuit voltage of the battery. Therefore, a device for estimating an open circuit voltage of a battery, by which an open circuit voltage of a battery can be relatively accurately estimated in a relatively short period of time after completion of a charge or discharge, can be provided.